Membrane separation process

ABSTRACT

Membrane separation process is improved by use, as a support membrane, of a polyacrylonitrile membrane the surface of which has been oxygen-plasma treated whereby at least a portion of the surface has been oxidized to --COOH groups.

RELATED APPLICATION

Application Ser. No. 07/425,156, filed Oct. 23, 1989 by Texaco Inc. asassignee of Craig R. Bartels, now U.S. Pat. No. 4,992,176 issued Feb. 2,1991.

FIELD OF THE INVENTION

This invention relates to a membrane separation process. Moreparticularly it relates to a membrane system characterized by itsimproved life when used to separate charge systems typified by aqueousmixtures of ethylene glycol or isopropanol.

BACKGROUND OF THE INVENTION

As is well known to those skilled in the art, it is possible to separatemixtures of liquids, typified by mixtures of water and organic liquidssuch as aqueous solutions of ethylene glycol or isopropanol, by varioustechniques including adsorption or distillation. These conventionalprocesses, particularly distillation, are however, characterized by highcapital cost. In the case of distillation for example, the processrequires expensive distillation towers, heaters, heat exchangers(reboilers, condensers, etc.), together with a substantial amount ofauxiliary equipment typified by pumps, collection vessels, vacuumgenerating equipment, etc.

Such operations are also characterized by high operating costsprincipally costs of heating and cooling--plus pumping, etc.

Furthermore the properties of the materials being separated, as isevidenced by the distillation curves, may be such that a large number ofplates may be required, etc. When the material forms an azeotrope withwater, additional problems may be present which for example, wouldrequire that separation be effected in a series of steps (e.g. as in twotowers) or by addition of extraneous materials to the system.

There are also comparable problems which are unique to adsorptionsystems.

It has been found to be possible to utilize membrane systems to separatemixtures of miscible liquids by pervaporation. In this process, thecharge liquid is brought into contact with a membrane film; and onecomponent of the charge liquid preferentially permeates the membrane.The permeate is then removed as a vapor from the downstream side of thefilm--typically by sweeping with a carrier gas or by reducing thepressure below the saturated vapor pressure of the permeating species.

Illustrative membranes which have been employed in prior art techniquesinclude those set forth in the following table:

                  TABLE                                                           ______________________________________                                        Separating Layer    References                                                ______________________________________                                        Nafion brand of     Cabasso and Liu                                           perfluorosulfonic acid                                                                            J. Memb. Sci. 24,                                                             101 (1985)                                                Sulfonated polyethylene                                                                           Cabasso, Korngold                                                             & Liu J. Pol. Sc:                                                             Letters, 23, 57                                                               (1985)                                                    Fluorinated polyether                                                                             USP 4,526,948                                             or Carboxylic Acid fluorides                                                                      to Dupont as assignee                                                         of Resnickto                                              Selemion AMV        Wentzlaff                                                 brand of Asahi Glass                                                                              Boddeker & Hattanbach                                     cross-linked styrene                                                                              J. Memb. Sci. 22, 333                                     butadiene (with quaternary                                                                        (1985)                                                    ammonium residues on a                                                        polyvinyl chloride backing)                                                   Cellulose triacetate                                                                              Wentzlaff, Boddeker                                                           & Hattanback, J. Memb.                                                        Sci. 22, 333 (1985)                                       Polyacrylonitrile   Neel, Aptel &                                                                 Clement Desalination                                                          53, 297 (1985)                                            Crosslinked         Eur. Patent 0 096                                         Polyvinyl Alcohol   339 to GFT as assignee                                                        of Bruschke                                               Poly(maleimide-     Yoshikawa et al                                           acrylonitrile)      J. Pol. Sci. 22, 2159                                                         (1984)                                                    Dextrine -          Chem. Econ. Eng.                                          isophorone diisocyanate                                                                           Rev., 17, 34 (1985)                                       ______________________________________                                    

The cost effectiveness of a membrane is determined by the selectivityand productivity. Of the membranes commercially available, anillustrative polyvinyl alcohol membrane of high performance is thatdisclosed in European patent 0 096 339 A2 of GFT as assignee ofBruschke-- published Dec. 21, 1983.

European Patent 0 096 339 A2 to GFT as assignee of Bruschke discloses,as cross-linking agents, diacids (typified by maleic acid or fumaricacid); dihalogen compounds (typified by dichloroacetone or1,3-dichloroisopropanol); aldehydes, including dialdehydes, typified byformaldehyde. These membranes are said to be particularly effective fordehydration of aqueous solutions of ethanol or isopropanol.

This reference discloses separation of water from alcohols, ethers,ketones, aldehydes, or acids by use of composite membranes. Specificallythe composite includes (i) a backing typically about 120 microns inthickness, on which is positioned (ii) a microporous support layer of apolysulfone or a polyacrylonitrile of about 50 microns thickness, onwhich is positioned (iii) a separating layer of cross-linked polyvinylalcohol about 2 microns in thickness.

Polyvinyl alcohol may be cross-linked by use of difunctional agentswhich react with the hydroxyl group of the polyvinyl alcohol. Typicalcross-linking agent may include dialdehydes (which yield acetallinkages), diacids or diacid halides (which yield ester linkages),dihalogen compounds or epichlorhydrin (which yield ether linkages)olefinic aldehydes (which yield ether/acetal linkages), boric acid(which yields boric ester linkages), sulfonamidoaldehydes, etc.

U.S. Pat. No. 4,992,176 which issued Feb. 12, 1991 to Texaco as assigneeof Craig R. Bartels is directed to separation of water from organicoxygenates, such as isopropanol, by use of a membrane system including asupport layer of polyacrylonitrile bearing a separating layer ofpoly(vinyl pyridine) which has been cross-linked with an aliphaticpolyhalide.

U.S. Pat. No. 4,728,429 to Cabasso et al, U.S. Pat. No. 4,067,805 toChiang et al, U.S. Pat. No. 4,526,948 to Resnick, U.S. Pat. No.3,750,735 to Chiang et al, and U.S. Pat. No. 4,690,766 to Linder et alprovide additional background.

Additional prior art which may be of interest includes:

Mobility of Spin Probes in Quaternized Poly(4-Vinylpyridine) Membranes,Makino, Hamada, and Iijima, in Polym. J. (Toyko), 19(6), 737-45, 1987.

Effect of Quaternization on the Pervaporation Rate of Water ThroughPoly(4-Vinylpyridine) Membrane, Hamaya, and Yamada, in KobunshiRonbunshu, 34(7), 545-7, 1977.

Preparation of Separation Membranes, Yamamoto, Toi, and Mishima, patent#JP 61/161109 A2, Jul. 21, 1986. (Japanese).

Separation of Some Aqueous Amine Solutions by Pervaporation throughPoly(4-Vinylpyridine) Membrane Yamada and Hamaya, in Kobunshi Ronbunshu,39(6), 407-14, 1982.

Complex Formation of Cross-linked Poly(4-Vinylpyridine) Resins withCopper (II), by Nishide, Deguchi, and Tsuchida, in Bulletin of theChemical Society of Japan, Vol. 49(12), 3498-3501 (1976).

Although many of these membrane systems of the prior art may exhibitsatisfactory Flux and Separation, it is found in practice that after themembrane assembly has been in use to effect a particular separation orfor an extended period of time, the assembly may tend to deteriorate andbecome brittle. In the membrane assembly of the above-noted U.S. Pat.No. 4,992,176 for example, it is found that mechanical stabilitydeteriorates to a degree that the Separation undesirably decreases.Although the length of time to reach this undesirable state may varydepending on the nature of the charge and the conditions of operation,it may occur in less than a few hours or in a few days.

Inspection of the membrane system reveals that deterioration is due tothe failure of the adhesion between the separating layer and the supportlayer. In the case for example of a polyvinyl pyridine separatingmembrane layer mounted on a polyacrylonitrile support, it is found thatthe bond therebetween has failed and this is evidenced by the visibleseparation of the layers as well as by the cracking of the separatingmembrane layer at those points at which the bond has failed.

It is an object of this invention to provide a membrane system,characterized inter alia by its ability to separate water from anorganic oxygenate typified by ethylene glycol, which possesses a highdegree of mechanical stability during such separation operations. Otherobjects will be apparent to those skilled in the art.

STATEMENT OF THE INVENTION

In accordance with certain of its aspects, this invention is directed toa membrane support layer, characterized by its high degree of bondingability to a membrane separating layer, comprising a membrane of acarbon-- carbon backbone polymer containing --CN groups and a surfacethereof which has been treated with oxygen-plasma whereby at least aportion of the surface has been oxidized to --COOH groups.

DESCRIPTION OF THE INVENTION

The composite structure of this invention includes a multi-layerassembly which in the preferred embodiment preferably includes a porouscarrier layer which provides mechanical strength and support to theassembly.

THE CARRIER LAYER

This carrier layer, when used, is characterized by its high degree ofporosity and mechanical strength. It may be fibrous or non-fibrous,woven or non-woven. In the preferred embodiment, the carrier layer maybe a porous, flexible, non-woven or woven fibrous polyester.

One typical non-woven polyester carrier layer may be formulated ofnon-woven, thermally-bonded strands and characterized by a fabric weightof 80±8 grams per square yard, a thickness of 4.2±0.5 mils, a tensilestrength (in the machine direction) of 31 psi and (in cross direction)of 10 psi, and a Frazier air permeability of 6 cuft/min/sq. ft. @ 0.5inches of water.

THE POROUS SUPPORT LAYER

The porous support layer of this invention is preferably formed of amembrane of a carbon-carbon backbone polymer containing --CN groups anda surface thereof bearing pendant carboxyl (--COOH) groups. This layermay be formed of polyacrylonitrile, at least a portion of the surface ofwhich has been converted to --COOH groups by oxygen plasma treatment.

When the porous support layer is formed from a polyacrylonitrile, thepolymer may typically be of molecular weight M_(n) of 5,000-100,000preferably 20,000-60,000, say 40,000. It may be the cast from a 5 w %-20w %, say 15 w % solution thereof in inert solvent (typically a solventsuch as dimethyl formamide, dimethyl sulfone, or dimethyl acetamide) toform a layer of 40-80 microns, say 50 microns thick. The cast membraneis then immersed in water at 4°-5° C. to 30° wherein the solvent isextracted and the membrane sets up. Solvent such as that in which thepolyacrylonitrile is dissolved, may also be added separately to thewater bath to aid in membrane formation. The membrane is then rinsedwith water to remove solvent.

In practice of the process of this invention according to certain of itsaspects, the so-prepared porous support layer, which is formed of thecarbon-carbon backbone bearing --CN groups which characterizepolyacrylonitrile, is treated to convert at least a portion of thesurface to --COOH groups by oxygen plasma treatment.

Although it may be possible to effect treatment of the polyacrylonitrilein solution prior to casting of the film or to attain the desiredresults by treatment of a mixture (in solvent) of a polyacrylonitrileand a polyacrylic acid, it is preferred to prepare the desired supportby casting a polyacrylonitrile membrane and then treating the surface toeffect oxidation of the surface --COOH groups.

Oxygen plasma treatment, in accordance with this invention, may becarried out by subjecting the support polymer, preferably in membraneform, to radio frequency energy in the presence of an oxygen-containingatmosphere. Although the oxygen-containing atmosphere may be air orenriched air, it is preferred to utilize an atmosphere containing atleast 99% preferably 99.9%-99.999%, say 99.99% oxygen. The temperatureof operation may be 0° C.-50° C., preferably 25° C.-35° C., say 25° C.at pressure of 20-300, say 25 millitors.

The radio frequency energy may be of frequency of 10 Hz-1OO MHz,preferably 1-60 MHz, say 13.56 MHz. It should be noted that in theUnited States, the Federal Communications Commission has assigned afrequency of 13.56 MHz to this type of operation; and accordinglyoperations in the United States should be confined to this frequency.The power employed may depend on the size of the system being treated.For treatment of membrane, it may be desirable to utilize 10-1000,preferably 100-1000, say 100 watts per square centimeter of membranesurface area. For a laboratory experimental membrane specimen it isconvenient to operate at a power level of about 100 watts. Clearly thetime of treatment may depend on the power level and the degree ofoxidation desired. Typically operation for 20-120 seconds may be foundto give desired results.

During such oxygen plasma treatment, no visible effect is noted. FTIRspectroscopy of the surface shows a broad band at 1749-1690 cm⁻¹indicating the presence of the --COOH functional group. X-rayphotoelectron spectroscopy on the surface shows an increase in theoxygen content from an initial value of 5%-9%, say 8% to a final valueof 11%-14%, say 11%; and commonly 2%-6%, say 3% higher than thatorginally present--which indicates presence of --COOH groups.

During this treatment, it may typically be found that the treatedsurface bears --COOH groups.

It is a feature of these several embodiments that they are characterizedby the same mass properties of a polyacrylonitrile membrane (withrespect e.g. to separation ability etc.) while simultaneously possessingaugmented bonding properties in the membrane system because of themodified surface characteristics generated by the treatment of theinstant invention.

THE SEPARATING LAYER

There is then deposited on the 40-80 micron thick so-treated supportlayer, the separating layer. The separating layer may be any of a widerange of membranes depending on the charge to be separated and theconditions of separation. It might for example be (i) a cross-linkedpolyvinyl alcohol membrane (ii) a quaternary ammonium-exchangedfluorinated ion exchanged membrane, (iii) a sulfonated polyethylenemembrane, (iv) a silicone or silicone/polycarbonate membrane, (v) across-linked polyimine membrane, etc.

A preferred separating layer or membrane which permits attainment ofseparation in accordance with this invention includes a non-porous filmof cross-linked poly(vinyl pyridine) of thickness of about 1-10 microns,preferably 1-5 microns, say 3 microns. This layer is formed (preferablyby casting) from a poly(vinyl pyridine) solution. Although poly(2-vinylpyridine) may be employed, the preferred separating layer is preparedfrom poly(4-vinyl pyridine)--typically the Reilline 4200 brand (ofReilly Tar and Chemical Co) of poly(4-vinyl pyridine) in a 10 w %solution in a suitable alcohol solvent such as methanol.

The separating membrane may be formed by mixing 0.5-2 parts, say 1 partof the 10%-30%, say 10 w % solution of poly(4-vinyl pyridine) inmethanol with 1 part methanol, and 0.1-0.8 parts, say 0.52 parts ofaliphatic polyhalide cross-linking agent and casting the mixture on asupport.

The separating layer may be a homopolymer or a copolymer of 2-vinylpyridine or more preferably 4-vinyl pyridine. When copolymers areemployed, the co-monomer may be an unsaturated monomer, typically vinylchloride, ethylene, vinyl alcohol, styrene, vinyl acetate, ethyleneoxide, etc. In the preferred embodiment, the separating layer is ahomopolymer of 4-vinyl pyridine of molecular weight M_(v) of10,000-500,000, preferably 100,000-300,000, say about 200,000.

The polymer may be cross-linked with a cross-linking agent to form themembranes useful in practice of this invention.

Typically the cross-linking agents may contain an aliphatic moiety,preferably containing 2-12 carbon atoms, typically 3-6 carbon atoms, say4 carbon atoms. Although the cross-linking agent may be a polyhalide, ittypically contains 2-5 halogen atoms, most preferably 2. The halogen ispreferably bromine or less preferably chlorine or iodine. The halidesmay preferably be alpha, omega dihalides of aliphatic hydrocarbons.Typical cross-linking agents may be as tabulated infra, the first listedbeing preferred:

                  TABLE                                                           ______________________________________                                        1,4-dibromo-n-butane   (DBB)                                                  1,5-dibromo-n-pentane  (DBP)                                                  1,3-dibromo propane                                                           1,6-dibromo hexane                                                            1,8-dibromo octane                                                            1,4-dichloro-n-butane                                                         ______________________________________                                    

In situ cross-linking may be carried out by casting onto the preferredtreated polyacrylonitrile support the poly(4-vinyl pyridine) typicallyin the solution in methanol to which has been added the cross-linkingagent (typically 1,4-dibromobutane) in mole ratio of cross-linking agentto polymer of 0.2-2, say about 1.13.

It may be possible in one embodiment to crosslink the poly(4-vinylpyridine) separating layer in one step by casting the solution ofpoly(4-vinyl pyridine) and polyhalide, followed by heat curing the castmembrane at 100° C.-200° C., say 125° C. for 1-30 minutes, say 2minutes.

In another embodiment, it may be possible to apply to the treated poroussupport layer, a solution of poly(4-vinyl pyridine). This may be driedat 40° C.-80° C., say 50° C. for 2-10 minutes, say 4 minutes to form afilm. There may then be added onto the surface of this film a solutionin methanol containing polyhalide and 2-7 w %, say 3.5 w % ofpoly(4-vinyl pyridine).

The composite membrane, whether prepared by the one-step or the two-stepprocess may then be cured in an oven at 100° C.-200° C., say 125° C. for1-30 minutes, say 2 minutes to yield a film having a thickness of 1-10microns, say 4 microns.

THE COMPOSITE MEMBRANE

It is a feature of this invention that the composite membrane maycomprise (i) an optional carrier layer, characterized by porosity andmechanical strength, for supporting a porous support layer and aseparating layer, (ii) as a porous support layer a membrane having acarbon-carbon backbone polymer containing --CN groups and a surfacethereof which has been treated with oxygen-plasma whereby at least aportion of the surface has been oxidized to --COOH groups, of molecularweight M_(n) of 5,000-100,000, of thickness of 10-80 microns, and ofmolecular weight cut-off of 25,000-100,000, and (iii) a non-porousseparating layer, preferably of poly(vinyl pyridine) of molecular weightM_(v) of 10,000-500,000 which has been cross-linked with an aliphaticpolyhalide.

The composite membranes of this invention may be utilized in variousconfigurations. It is, for example, possible to utilize the composite ina plate-and-frame configuration in which separating layers may bemounted on the porous support layer with the carrier layer.

It is possible to utilize a spiral wound module which includes anon-porous separating layer membrane mounted on a porous support layerand a carrier layer, the assembly being typically folded and bonded orsealed along all the edges but an open edge--to form a bag-like unitwhich preferably has the separating layer on the outside. A clothspacer, serving as the permeate or discharge channel is placed withinthe bag-like unit. The discharge channel projects from the open end ofthe unit.

There is then placed on one face of the bag-like unit, adjacent to theseparating layer, and coterminous therewith, a feed channelsheet--typically formed of a plastic net.

The so-formed assembly is wrapped around a preferably cylindricalconduit which bears a plurality of perforations in the wall--preferablyin a linear array which is as long as the width of the bag-like unit.The projecting portion of the discharge channel of the bag-like unit isplaced over the performations of the conduit; and the bag-like unit iswrapped around the conduit to form a spiral wound configuration.

It will be apparent that, although only one feed channel is present, thesingle feed channel in the wound assembly will be adjacent to two facesof the membrane layer. The spiral wound configuration may be formed bywrapping the assembly around the conduit a plurality of times to form areadily handleable unit. The unit is fitted within a shell (in mannercomparable to a shell-and-tube heat exchanger) provided with an inlet atone end and an outlet at the other. A baffle-like seal between the innersurface of the shell and the outer surface of the spiral-wound inputprevents fluid from bypassing the operative membrane system and insuresthat fluid enters the system principally at one end. The permeate passesfrom the feed channel, into contact with the separating layer and thencetherethrough, into the permeate channel and thence therealong to andthrough the perforations in the conduit through which it is withdrawn asnet permeate.

In use of the spiral wound membrane, charge liquid is permitted to passthrough the plastic net which serves as a feed channel and thence intocontact with the nonporous separating membranes. The liquid which doesnot pass through the membranes is withdrawn as retentate. The liquid orvapor which permeates the membrane passes into the volume occupied bythe permeate spacer and through this permeate channel to theperforations in the cylindrical conduit through which it is withdrawnfrom the system. In this embodiment, it will be apparent that the systemmay not include a carrier layer.

In another embodiment, it is possible to utilize the system of thisinvention as a tubular or hollow fibre. In this embodiment, the poroussupport layer of e.g. polyacrylonitrile may be extruded as a fine tubewith a wall thickness of typically 0.001-0.1 mm. The extruded tubes aresubjected to oxygen-plasma treatment and then passed through a bath ofe.g. poly(vinyl pyridine) in n-butanol which then is cross-linked andcured. A bundle of these tubes is secured (with an epoxy adhesive) ateach end in a header; and the fibres are cut so that they are flush withthe ends of the header. This tube bundle is mounted within a shell in atypical shell-and-tube assembly.

In operation, the charge liquid is admitted to the tube side and passesthrough the inside of the tubes and exits as retentate. During passagethrough the tubes, permeate passes through the non-porous separatinglayer and permeate is collected in the shell side.

In this embodiment, it will be apparent that the system may not normallyinclude a carrier layer.

PERVAPORATION

It is a feature of the membrane assembly including the membrane supportlayer and the non-porous separating layer mounted thereon that, althoughthis system may be useful in various membrane processes includingreverse osmosis, it is found to be particularly effective when used in apervaporation process. In pervaporation, a charge liquid containing amore permeable and a less permeable component is maintained in contactwith a non-porous separating layer; and a pressure drop is maintainedacross that layer. The charge liquid dissolves into the membrane anddiffuses therethrough. The permeate which passes through the membraneand exits as a vapor may be recovered by condensing at low temperatureor alternatively may be swept away by use of a moving stream of gas.Preferably, the permeate side of the membrane is maintained at a lowpressure, typically 5 mm. Hg.

For general background on pervaporation, note U.S. Pat. No. 4,277,344;U.S. Pat. No. 4,039,440; U.S. Pat. No. 3,926,798; U.S. Pat. No.3,950,247; U.S. Pat. No. 4,035,291; etc.

It is a feature of the process of this invention that the novel membranemay be particularly useful in pervaporation processes for dewateringaqueous mixtures of organic oxygenates. It may be possible to utilizethe process of this invention to remove water from immiscible mixturestherewith as in the case of ethyl acetate (solubility in water at 15° C.of 8.5 parts per 100 parts of water). It will be apparent to thoseskilled in the art that it may be desirable to separate large quantitiesof water from partially miscible systems as by decantation prior toutilizing the process of the invention to remove the last traces ofwater.

The advantages of the instant invention are more apparent when thecharge liquid is a single phase homogeneous aqueous solution as is thecase for example with aqueous solutions of isopropanol or ethyleneglycol. The system may also find use in the case of slightly solubleliquids wherein two phases are present (i) water-oxygenate first phaseand, as a second phase (ii) either water or oxygenate. Clearly thosecharge liquids which contain only a small portion of an immisciblesecond liquid phase may benefit most from the process of this invention.It is also a feature of this invention that it may be particularlyuseful to separate azeotropes such as isopropanol-water.

It is a particular feature of this invention that use of the membranesystem (preferably employing the treated--surface polyacrylonitrilesupport) permits attainment of separation systems which possess all theadvantages attained using untreated polyarylonitrile in addition tosustantially improved mechanical and chemical stability.

The charge organic oxygenates which may be treated by the process ofthis invention may include alcohols, glycols, weak acids, ethers,esters, ketones, aldehydes, etc. It will be apparent to those skilled inthe art that the charge organic oxygenates used should be inert withrespect to the separating membrane. Clearly a system wherein themembrane is attacked by the components of the charge liquid will notyield significant separation for any reasonable period of time. Bestresults may be achieved when treating alcohols (such as isopropanol) orglycols (such as ethylene glycol). Results achieved with acids aregenerally less satisfactory.

Illustrative alcohols may include ethanol, propanol, i-propanol,n-butanol, i-butanol, t-butanol, amyl alcohols, hexyl alcohols, etc.

Illustrative glycols may include ethylene glycol, propylene glycols,butylene glycol or glycol ethers such as diethylene glycol, triethyleneglycol, or triols, including glycerine; etc.

Illustrative chlorinated hydrocarbons may include dichloroethane,methylene dichloride, etc.

Illustrative weak acids may include hexanoic acid, octanoic etc. (Whenacids are present, preferably the pH of the charge liquid should beabove about 4. Typical acids which may be treated by the process of thisinvention include those having a pKa≧ca 4.8.

Illustrative esters may include ethyl acetate, methyl acetate, butylacetate, methyl benzoate, ethylene glycol mono acetate, propylene glycolmonostearate, etc.

Illustrative ethers may include tetrahydroforan, diethyl ether,diisopropyl ether, etc.

Illustrative ketones may include acetone, methyl ethyl ketone,acetophenone, etc.

Illustrative aldehydes may include formaldehyde, acetaldehyde,propionaldehyde, etc.

It is believed that the advantages of this invention are most apparentwhere the organic oxygenate is a liquid which is infinitely misciblewith water--typified by isopropyl alcohol or ethylene glycol.

A typical charge may be an aqueous solution containing 70%-95%, say 85 w% isopropanol.

In practice of the pervaporation process of this invention, the chargeaqueous organic oxygenate solution typically at 40° C.-120° C., say 80°C. may be passed into contact with the non-porous separating layer ofthe membrane of this invention. A pressure drop of about one atmosphereis commonly maintained across the membrane. Typically, the feed orcharge side of the membrane is at about atmospheric pressure and thepermeate or discharge side of the membrane is at a pressure of about2-50 preferably 5-20, say 10 mm. Hg.

The permeate which passes through the membrane includes water and asmall proportion of the organic oxygenate from the charge liquid.Typically, the permeate contains 70-99.5, say 75 w % water. Permeate isrecovered in vapor phase.

Performance is judged by the ability of a membrane system to give apermeate containing decreased content of organic oxygenate (from acharge containing a higher content of organic oxygenate and water) witha good flux (kilograms/meter² -/hour (kmh)) at a predetermined feedtemperature and with a vacuum on the permeate side and a condenser(cooled by liquid nitrogen). Compositions falling outside the scope ofthis invention may be characterized by unsatisfactory separation orunsatisfactory productivity (flux) or both.

Pervaporation may typically be carried out at a flux of about 0.3-2, say1.7 kilograms per square meter per hour (kmh). Typically, the units mayshow good separation (measured in terms of w % organic oxygenate in thepermeate during pervaporation of an aqueous solution of organicoxygenate through a poly(4-vinyl pyridine) separating layer.

It will be noted that as the concentration of the charge increases, theconcentration of oxygenate in the permeate increases and the Fluxdecreases.

Practice of the process of this invention will be apparent to thoseskilled in the art from inspection of the following examples wherein, aselsewhere in this specification, all parts are parts by weight unlessotherwise stated. An asterisk indicates a control example.

DESCRIPTION OF SPECIFIC EMBODIMENTS Example I

In this example which represents the best mode presently known ofcarrying out the process of this invention, the porous carrier layeremployed is the DUY-L brand of nonwoven polyester of the DaicelCorporation. The porous support layer is a microporous polyacrylonitrilemembrane layer of molecular weight M_(n) of 40,000. This layer, mountedon the porous carrier layer, (in a circular assembly of 7.62 cmdiameter) is subjected to oxygen plasma etching in a reaction chambercontaining oxygen of 99.9% purity at 25° C. and 25 millitors. Plasmatreating is effected at 13.56 MHz applied at a power of 100 watts andthe surface is found to bear --COOH groups. This is evidenced by thepresence of the broad peak at 1749-1688 cm⁻¹ in the infra red (FTIR),which corresponds to carboxylic acid stretching. The product containssurface --CN groups and surface --COOH groups.

This support is coated with poly(4-vinyl pyridine) by contact with asolution containing 2.5 parts of n-butanol, 2.5 parts of 20%poly(4-vinyl pyridine) in methanol, and 1.3 parts of 1,4-dibromobutane.This 2-mil coating is dried at 125° C. for 2 minutes and cured at 125°C. for two minutes.

This membrane system is used to separate a charge liquid containing 85 w% ethylene glycol and 15 w % water. Charge is admitted to thepervaporation cell at 70° C. The permeate condenser yields an aqueoussolution containing 86 w % water and only 14 w % ethylene glycol. TheFlux is 1.7 kmh and the permeate contains ca 86 w % water.

After two days of operation, the membrane is inspected. It exhibits noevidence of separation, brittleness, or cracking. The surface is assmooth as the original.

Example II*

In this control example, the procedure of Example I is duplicated exceptthat the polyacrylonitrile is not treated by the oxygen-plasmatechnique.

On disassembly and inspection after 3 hours of operation, it is foundthat the poly(vinyl pyridine) separating layer is brittle and cracked;and it has mechanically separated from the polyacrylonitrile supportlayer.

Example III

In this example, the procedure of Example I is carried out except thatthe polyvinylpyridine is cross-linked, not with 1,4-dibromobutane, butwith an equal weight of 1,6-dibromohexane.

The resulting membrane shows Flux of 1.3 kmh and selectivity of 82% andno evidence of any mechanical separation after seven days. The permeatecontains 82% water.

Example IV*

In this control example, the support layer is formed (as in Example III)except that no oxygen-plasma treatment is employed.

The procedure of Example III is otherwise followed; and on dissembly andinspection often 3 hours, it is found that the poly (vinyl pyrrdine)separating layer is brittle and cracked and it has mechanicallyseparated from the polyacrylonitrile support layer.

Examples V-IX

Results comparable to those of Example I may be attained if theseparating membrane layer is:

                  TABLE                                                           ______________________________________                                        EXAMPLE      Separating Membrane Layer                                        ______________________________________                                        V            Polyvinyl alcohol of -- M.sub.n of 115,000                                    cross-linked with glutaraldehyde                                 pervaporation                                                                 VI           Nafion-H 117 Fluorinated ion                                     exchange membrane which has been                                                           exchanged with tetra-n-octyl                                                  ammonium bromide - pervaporation                                 VII          Sulfonated polyethylene which has                                             been contacted with LiCl                                         pervaporation                                                                 VIII         Polyethylene imine which has been                                             cross-linked with toluene                                                     diisocyanate - reverse osmosis                                   IX           A blend of polyvinyl alcohol and                                              polyacrylic acid                                                 pervaporation                                                                 ______________________________________                                    

Although this invention has been illustrated by reference to specificembodiments, it will be apparent to those skilled in the art thatvarious charges and modifications may be made which clearly fall withinthe scope of the invention.

What is claimed:
 1. A membrane support layer, characterized by its highdegree of bonding ability to a membrane separating layer, comprising acarbon-carbon backbone polymer containing --CN groups and a surfacethereof which has been treated with oxygen-plasma sufficient to enableat least a portion of the surface to be oxidized to --COOH groups.
 2. Amembrane layer of a polyacrylonitrile, which has been treated withoxygen-plasma sufficient to enable at least a portion of the surface tobe oxidized to --COOH groups.
 3. A membrane system, characterized by itsability to separate water from a charge containing water and an organicoxygenate, which comprises a membrane support layer of apolyacrylonitrile, which has been treated with oxygen-plasma sufficientto enable at least a portion of the surface to be oxidized to --COOHgroups; and mounted thereon a non-porous separating elastomer membranelayer.
 4. The method of treating a membrane layer of polyacrylonitrilewhich contains surface --CN groups which comprisestreating the surfaceof said polyacrylonitrile with oxygen plasma sufficient to enable atleast a portion of said surface to be oxidized to surface --COOH groupsso as to form a membrane layer containing surface --CN groups and atleast a portion of the surface of which has been oxidized to --COOHgroups; and recovering said membrane layer containing said surface atleast a portion of which has been oxidized to --COOH groups.
 5. Themethod of treating a charge containing water and organic oxygenate whichcomprisesmaintaining a membrane assembly including (i) a porous supportlayer of a polyacrylonitrile, a surface of which has been treated withoxygen plasma sufficient to enable at least a portion of the surface tobe oxidized to --COOH groups and (ii) a separating elastomer membranemounted on and bonded to said porous support layer; maintaining apressure drop across said nonporous separating elastomer membrane;passing a charge aqueous solution of an organic oxygenate into contactwith the high pressure side of said nonporous separating elastomermembrane whereby at least a portion of said water in said charge aqueoussolution and a lesser portion of organic oxygenate in said chargeaqueous solution pass by pervaporation through said non-porousseparating elastomer as a lean mixture containing more water and lessorganic oxygenate than are present in said charge aqueous solution andsaid charge aqueous solution is converted to a rich liquid containingless water and more organic oxygenate than are present in said chargeaqueous solution; recovering as permeate from the low pressure side ofsaid non-porous elastomer membrane, said lean mixture containing morewater and less organic oxygenate than are present in said charge aqueoussolution, said lean mixture being recovered in vapor phase at a pressurebelow the vapor pressure thereof; and recovering as retentate from thehigh pressure side of said non-porous separating membrane said richliquid containing a lower water content and a higher organic oxygenatecontent than are present in said charge aqueous solution.